The problem is in defining the constraints and node properties. There is a mismatch between the two,

CREATE CONSTRAINT ON(a:Activity) ASSERT a.activity_id IS UNIQUE;
In a.Activity node the property is defined as 'id' and hence the mismatch.
In the constraint it's a.activity_id and in the node it's a.id.
Replace 'a.activity_id' in the constraint with 'id'.

Thanks! I made those changes but it's still giving me an error after running the first CSV statement, "Neo.ClientError.Schema.ConstraintValidationFailed: Node(96) already exists with label Activity and property id = 'aaec265e3f3.1"

I've changed all the constraints so that they're referencing (node).id or (node).name - I don't THINK they should be reference (node).activity_id because 'activity_id' is just the header in the CSV, but the key for the property is id.